1. Field of the Invention
The present invention relates to an image reading apparatus adapted for reading an original image and for converting it into digital data.
2. Description of the Related Art
There has recently been an expectation that a chance to convert image information of a great volume of documents into electronic data and to store the data will increase even at an administrative institution. Besides, to utilize the converted electronic data as official documents which become legally valid, a need to convert paper documents into electronic data more faithfully has increased.
In the case of a conventional image reading apparatus, however, when a document with a correction trace by a whiteout or a falsified document is read, it is impossible to determine whether it is a corrected document or not from electronic data, thus reducing reliability of an original.
To detect a whiteout or the like on an original in the image reading apparatus of such a type, the following method has conventionally been available. According to this method, reflected light of light applied to the surface of a form (original) from a light source for reflected light is read by a first sensor, and transmitted light of light applied to the backside of the form from a light source for transmitted light is read by a second sensor. A difference is taken between outputs of the first and second sensors. When an occupation ratio of a black image in this difference image is larger than a predetermined value, application of a whiteout is determined (e.g., see Japanese Patent Application Laid-Open No. 2001-236463).
When an original having an uneven surface or a metal of a high surface reflectance is read, a method of preventing a whitish image caused by saturation of charges stored in a charge-coupled device (CCD) may be employed. According to this method, when the charges stored in the CCD are saturated, a driving voltage for an illumination halogen lamp is reduced to decrease a quantity of light applied to the original, thereby increasing an amplification degree of an output signal from the CCD (e.g., see Japanese Patent Application Laid-Open No. 07-038713).
However, even in the case of the configuration capable of detecting a whiteout, application of the whiteout is only detected, and the two light sources and the two sensors are necessary only for the detection, thus causing a problem of high costs. Furthermore, in the case of the technology for decreasing the quantity of light applied to an original, a void image caused by saturation of a CCD itself by a whiteout or the like is only prevented, thus causing a problem of impossible detection of a correction trace by the whiteout.